1. Field of the Invention
The present invention relates to a nonmagnetic toner for forming full color images, and specifically relates to a nonmagnetic toner for developing electrostatic latent images in full color image forming apparatuses such as full color electrostatic type copiers, full color laser beam printers and the like.
2. Description of the Related Art
Conventionally, when fixing a toner image on a transfer sheet by means of a contact type heat-fixing method such as a heating roller fixing method or belt fixing method, a disadvantage arises in that during development toner is melted and transferred to a fixing roller, fixing belt or the like via contact therewith, and the transferred toner is thereafter re-adhered to a transfer sheet causing image noise (offset phenomenon).
The inclusion of wax such as low molecular weight polypropylene in the toner is a known art for preventing offset phenomenon but this practice produces further disadvantages described below when this art is applied to toner used in full color image forming apparatuses.
For example, when forming full color images using a full color laser beam printer, it is necessary to reduce the dot size for high resolution color image reproduction, and the variable density of the colors of the full color image are reproduced by changing the dot surface ratio per unit surface area. When forming an image of a pale color having a low dot surface area ratio (i.e., an image formed of separate dots), diffused reflection caused by said dots reduces light transmittancy and color reproducibility and becomes markedly problematic as dot size becomes smaller compared to images of deep colors having a high dot surface area ratio (image formed of continuous dots). This problem becomes even more pronounced when wax is added to the toner, and markedly reduced color reproducibility of pale colors when forming images on overhead projector transparencies (hereinafter referred to as "OHP film"). The addition of wax to the toner also causes new disadvantages such as filming on the photosensitive member.
Full color image forming apparatuses of compact design have been studied in recent years, and require ever more compact developing devices. In full color image forming apparatuses, four developing devices are required to respectively accommodate cyan developer, yellow developer magenta developer, and black developer. Nonmagnetic monocomponent developing devices which do not require a mixing mechanism to mix the toner and carrier are particularly advantageous when reducing the size of the developing device, but in the case of nonmagnetic monocomponent developing devices there is a disadvantage of toner readily adhering to the developer carrying member and developer regulating member. This problem of undesirable toner adhesion is particularly pronounced in toners used for full color developing which employ binder resin having sharp melt characteristics during fixing which are necessary for light transmittancy.